Exhaust gas system for a motor vehicle

ABSTRACT

An exhaust-gas system for a motor vehicle includes at least one exhaust-gas cleaning system and at least one regeneration device for the exhaust-gas cleaning system, by means of which regeneration device an oxidizable fluid in vapor form can be introduced into the exhaust-gas stream ahead of the exhaust-gas cleaning system. The regeneration device includes a housing with a chamber formed in it as well as a heating element positioned in the chamber, the housing having on its upstream side an inflow opening for exhaust gases and on its downstream side an outlet opening and being arranged in the interior of the exhaust-gas stream.

The invention relates to an exhaust-gas system for a motor vehicle, comprising at least one exhaust-gas cleaning system and at least one regeneration device for the exhaust-gas cleaning system, by means of which regeneration device an oxidizable fluid can be introduced into the exhaust-gas stream ahead of the exhaust-gas cleaning system, the regeneration device comprising a housing with a chamber formed in it as well as a heating element positioned in the chamber. Exhaust-gas cleaning systems in the sense of the invention may include, in particular, diesel particle filters and NO_(x)-storage catalytic converters.

In order to observe environmental guidelines it is required to clean the exhaust-gases of vehicles which are driven by an internal combustion engine. For example, suitable particle filters are employed for reducing the particle-shaped emissions of diesel motor vehicles. Such particle filters have to be regenerated from time to time through burning off the particles which have accumulated on the filter surface. To this end, an oxidation-based catalytic converter is usually arranged upstream of the diesel particle filter, which through oxidation of an oxidizable substance in the exhaust-gas stream produces the heat which is needed for burning off the diesel soot particles.

Furthermore, NO_(x)-storage catalytic converters are increasingly used, which serve for reducing the nitric-oxide emission. For regenerating such storage catalytic converters it is required to enrich the exhaust-gas. For this, special measures are required in particular in the case of diesel engines which run with an excess of air and whose exhaust-gas likewise has an air surplus.

Thus, for regenerating a diesel particle filter or NO_(x)-storage catalytic converter in the exhaust-gas system of a lean-combustion engine it is required from time to time to enrich the exhaust-gas with an oxidizable substance. To this end, systems are known which use an additional high pressure fuel injection valve which is arranged in the manifold region of the exhaust-gas system and injects fuel directly into the exhaust-gas stream. These systems have the disadvantage, however, that a sufficient mixing of the liquid fuel with the exhaust-gas is not always guaranteed.

Moreover, mechanisms are known (see e.g. EP 1 369 557 A1) in which a defined amount of fuel is introduced into the heating chamber of a vaporizer which is arranged outside the exhaust-gas stream, where the fuel is vaporized and subsequently is introduced into the exhaust-gas stream. For vaporizing the liquid fuel an electrical glow plug is used in most cases, which has a comparably high energy consumption.

In contrast to this, the invention provides an exhaust-gas system with a regeneration device which compared to the systems known from prior art provides for a more uniform distribution of the oxidizable fluid and at the same time needs a smaller amount of electric energy.

According to the invention, in an exhaust-gas system of the type initially mentioned provision is made that the housing has on its upstream side an inflow opening for exhaust gases and on its downstream side an outlet opening and is arranged in the interior of the exhaust-gas stream. Thus, the exhaust-gas system according to the invention possesses a regeneration device which is arranged within an exhaust-gas carrying tube and through which a part of the exhaust-gas flows. Provided in the housing and upstream of the heating element is an injection nozzle for the oxidizable fluid, which sprays the fluid in the direction of the exhaust-gas stream onto the heating element. The arrangement of the regeneration device in the interior of the exhaust-gas stream offers the advantage here that the heating element is heated not only by electric energy, but in addition by the exhaust-gases flowing through the chamber of the regeneration device. Thereby the energy is reduced which is required for vaporizing the fluid. In addition, the exhaust-gases flowing through the housing help to transport the fluid through the heating element. Compared with an arrangement of a heating element with an upstream fluid injection nozzle directly in the exhaust-gas carrying tube, i.e. without a housing which surrounds the regeneration device, the configuration according to the invention is distinguished in that a more complete vaporization is achieved in the chamber by the protecting housing.

The housing preferably has an essentially tubular shape with a tapered upstream end. In this way it is achieved that merely a small part of the available exhaust-gas flows through the housing, which furthermore has a comparably small resistance to flow. What is more, due to the continuous widening of the housing in the direction of flow of the exhaust-gas, with such widening existing at least in a first portion of the housing, the flow velocity of the gas in the interior of the housing is diminished. Thereby the dwell time of the exhaust-gas in the chamber will be increased, likewise resulting in a more complete vaporization of the injected fluid. Designing the downstream housing section as a cylindrical tube with an essentially perpendicularly cut end provokes a turbulent flow in the region of the outlet opening, whereby a particularly effective mixing of the enriched gas from the regeneration device with the remaining exhaust-gas stream is ensured.

The heating element advantageously includes a porous foam. The latter has a particularly high surface area which is effective for vaporization, allowing a high heat transfer from the heating element to the fluid which is to be vaporized. Also the dwell time of the fluid in the region of the heating element will be correspondingly increased.

The foam may be a ceramic foam or a metallic foam. For heating, the latter may be wrapped by heating wires or interspersed with these, but it is also possible to apply a voltage directly to the porous material.

An additional fluid supply in the vehicle can be dispensed with, if the fluid is a fuel.

Further features and advantages of the invention will be apparent from the following description with the aid of the attached drawing in which:

FIG. 1 shows a schematic partial view of an exhaust-gas system according to the invention; and

FIG. 2 is an enlarged illustration of a regeneration device, as it is used in the exhaust-gas system of FIG. 1.

FIG. 1 shows a section of an exhaust-gas system 10 which is arranged downstream of a diesel engine of a motor vehicle (not shown). The exhaust-gas system 10 has a first tube section 14 which can be coupled to a manifold through a flange 12 as well as an exhaust-gas cleaning system 16 in the form of a diesel particle filter with an upstream or integrated oxidation-based catalytic converter or in the form of a NO_(x)-storage catalytic converter. The exhaust-gas cleaning system 16 is connected through a second tube section 18 with an end portion (not shown) of the exhaust-gas system 10. Of course, the exhaust-gas system according to the invention may have further exhaust-gas cleaning elements, for instance an oxidation-based catalytic converter which is arranged near the engine, or, in the case of a combination of NO_(x)- and particle reduction, a SCR-catalytic converter.

For regenerating the exhaust-gas cleaning system 16, a regeneration device 20 is provided which is arranged in the interior of the first tube section 14 immediately upstream of the exhaust-gas cleaning system 16. The regeneration device 20 comprises an essentially tubular housing 22 which at its upstream side is conically tapered and has a small inflow opening 24 for exhaust gases (see FIG. 2). At the downstream side of the housing 22, an outlet opening 26 is provided which extends across almost the entire end face of the housing 22.

A chamber 28 is formed in the interior of the housing 22; an injection nozzle 30 for fuel extends into this chamber. The injection nozzle is connected with a fuel line of the motor vehicle through a (not shown) fuel pump or metering device. The downstream part of the chamber 28 is almost completely occupied by a heating element 32 which is a porous ceramic foam or metallic foam. For the purpose of heating, this foam is wrapped by a current-carrying wire or interspersed by it, or a voltage is applied to the (electrically conductive) foam itself. As an alternative, also a plate-shaped honeycomb-like ceramic structure may be provided as the heating element 32, which structure has the shape of the internal diameter of the housing 22. The important point in the selection of the heating element 32 is merely that it has a structure which is distinguished by an as large a surface area as possible and which offers the gas, which flows through it, a certain resistance which is not excessively high.

For initiating the regeneration, some fuel is injected through the injection nozzle 30 into the chamber 28 towards the heating element 32. This fuel impinges on the heating element 32 which has a particularly high surface area for vaporizing the fuel, and becomes vaporized. At the same time, a part of the exhaust gas (in FIG. 2 illustrated by arrows) flows through the inflow opening 24 into the chamber 28, through the pores or openings of the heating element 32 and finally, entraining the fuel which now is in the vapor state, through the outlet opening 26 out of the housing 22. Thus, a part of the exhaust-gas is used to transport the injected fuel through the heating element 32. Further, the thermal energy of the exhaust-gas which flows through the chamber 28 is used for additionally heating the heating element, whereby the electric energy which is needed for vaporizing the fuel can be advantageously lowered. Thus, electrically produced thermal energy as well as the thermal energy of the exhaust-gas is used for initiating the regeneration of the exhaust-gas cleaning system 16.

In consequence of the special shape of the housing 22 with the chamber 28 which initially widens in the direction of flow, the flow velocity of the exhaust-gas reduces in the interior of the chamber 28. In this way a longer dwell time of the exhaust-gas in the chamber 28 is achieved, whereby a complete vaporizing of the supplied fuel is ensured. In addition, the shape of the housing 22 and its arrangement parallel to the outer wall of the first tube section 14 provides for a particularly good mixing of the exhaust-gas, coming from the chamber 28 and enriched with fuel, with the exhaust gas which flows past the housing 22 outside the latter. It is in particular the design of the tubular housing 22 in the region of the outlet opening 26 with the abrupt end which provides for strong turbulences downstream of the housing 22 (indicated by the eddies in FIG. 2), which allow an optimum distribution of the supplied fuel. The difference in pressure between the gas flows due to the differing flow velocities in the interior of the chamber 28 and outside the housing also makes a contribution to this. 

1-7. (canceled)
 8. An exhaust-gas system for a motor vehicle, comprising: at least one exhaust-gas cleaning system, and at least one regeneration device for the exhaust-gas cleaning system, the regeneration device being configured to introduce an oxidizable fluid in vapor form into the exhaust-gas stream ahead of the exhaust-gas cleaning system, wherein (i) the regeneration device comprises a housing with a chamber formed therein and a heating element positioned in the chamber, (ii) the housing has, on its upstream side, an inflow opening for exhaust gases and on its downstream side an outlet opening, (iii) the housing is arranged in the motor vehicle's exhaust-gas stream, and (iv) the housing has an essentially tubular shape with a tapered upstream end.
 9. The exhaust-gas system according to claim 8, wherein the heating element comprises a porous foam.
 10. The exhaust-gas system according to claim 9, wherein the foam is a ceramic foam.
 11. The exhaust-gas system according to claim 9, wherein the foam is a metallic foam.
 12. The exhaust-gas system according to claim 8, wherein the fluid is a fuel.
 13. The exhaust-gas system according to claim 8, wherein the exhaust-gas cleaning system is a diesel particle filter.
 14. The exhaust-gas system according to claim 8, wherein the exhaust-gas cleaning system is a NO_(x)-storage catalytic converter. 